Two Years of Nonthermal Emission from the Binary Neutron Star Merger GW170817: Rapid Fading of the Jet Afterglow and First Constraints on the Kilonova Fastest Ejecta

We present Chandra and Very Large Array observations of GW170817 at ∼521-743 days post-merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time nonthermal emission follows the expected evolution of an off-axis relativistic jet, with a steep temporal decay Fν ∝ t-1.95±0.15 and power-law spectrum Fν ∝ ν-0.575±0.007. We present a new method to constrain the merger environment density based on diffuse X-ray emission from hot plasma in the host galaxy and find n ≤ 9.6 × 10-3. This measurement is independent from inferences based on jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy E0 = 1.5-1.1+3.6 × 1049 erg (Eiso = 2.1-1.5+6.4 × 1052 erg) and jet opening angle θ0 = 5.9-0.7+1.0 deg with characteristic Lorentz factor Γj = 163-43+23, expanding in a low-density medium with n0 = 2.5-1.9+4.1 × 10-3 cm-3 and viewed θobs = 30.4-3.4+4.0 deg off-axis. The synchrotron emission originates from a power-law distribution of electrons with index p = 2.15-0.02+0.01. The shock microphysics parameters are constrained to ϵe = 0.18-0.13+0.30 and ϵB = 2.3-2.2+16.0 × 10-3. Furthermore, we investigate the presence of X-ray flares and find no statistically significant evidence of ≥2.5σ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy EkKN ∝ (Γ β)-α into the environment, finding that shallow stratification indexes α ≤ 6 are disfavored. Future radio and X-ray observations will refine our inferences on the fastest kilonova ejecta properties.